I did some checking. Using TaxCut2000 and inputing a married couple, ages 63 & 67 with a combined SS income of 20K and only 100% taxable income from an IRA of 43.1k and 43K I show a tax of 6454 and 6431 which yields an incremental rate of 23%.

Now when I change the inputs to an SS income of 19K and IRA income of 44.1K and 44K it shows a tax of 6753 and 6697. This shows an increase in taxes of $56 for the additional $100 of income.

SS income is an important factor when looking at the effective marginal tax rate.